Catch basins are widely used for the collection of drainage water from various sources. They are placed below grade so as to receive gravity flow. Customarily they have an open top that is closed by a rigid grate which often is subject to heavy bearing loads such as from vehicles which ride over the grate. The grate in turn transmits the load to the catch basin structure itself through the rim of the basin. The structural problem is complicated by the fact that the walls of the catch basin are usually weakened by openings through them that are provided to receive and to pass water received from the grate and from conduits connecting the basin to some other source or destination.
Catch basins and their grates have generally been made from materials of general use in the construction industry, such as concrete for heavy walls and steel for strong grates. Organic plastic materials have many properties which should recommend them for use in catch basins, such as lightness of weight, convenience in installation, and ultimate lesser installed cost. However, the shapes required for the function and load-resisting properties of the catch basin with the use of organic plastics have not previously been economically attainable with conventional casting and molding techniques. This is especially the situation for larger part sizes of 18-24 inches across.
Conventional processes require male and female mold parts. These cannot form integral structures having shapes that will withstand the loads because they generally preclude the removal of the male mold portion. Such shapes can be made from a plurality of cast or molded parts, but then these must be joined together, and there are inherent discontinuities where they are joined by various techniques, all of which involve cost.
In addition, the costs of the conventional mold parts themselves, and the machinery required to use them, are very large. If a catch basin made of organic plastic material is to be made economically and structurally satisfactorily, a different process must be used.
One such process is rotational molding. Because its only requirement is that all surfaces, both internal and external, be defined by the inside wall of a closed mold, the cost of the mold is dramatically reduced in comparison with a two or more piece mold with both male and female portions, and often at least several pull axes. It does, however, require a resourceful product design to withstand the loads, and which can be formed only by reference to the inside wall of a mold.
Evidently there will be a separation line in the rotational molding process so the rotational mold can be opened, but when it is opened, a complete single piece remains. The process itself involves rotating the mold around two or more axes in a mold which is heated from the outside. The proper amount of solid granular plastic material is placed in the mold, and as it is heated and rotated, a layer of material is deposited and cured on the inside wall, which ultimately constitutes the product. The thickness of the wall is established by the amount of plastic material that is used.
It is an object of this invention to provide a catch basin made of one continuous body of organic plastic which will withstand heavy bearing loads exerted by an overlaying grate, and which can be manufactured by a straight-forward and economical rotational molding process.